Any discussion of the prior art throughout this specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Photoluminescence (PL) imaging, performed for example using apparatus and methods disclosed in PCT Patent Application Publication No WO 2007/041758 A1 entitled ‘Method and System for Inspecting Indirect Bandgap Semiconductor Structure’ and incorporated herein by reference, has been shown to be of value for the rapid characterisation of silicon materials and devices, and silicon wafer-based solar cells in particular. As shown schematically in FIG. 1, luminescence 2 generated from a semiconductor sample 4 with broad area photo-excitation from a source 6 of above-bandgap light 8 can be imaged with a camera or CCD array 10 via collection optics 11, with the system preferably including homogenisation optics 12 to improve the uniformity of the broad area excitation and a long-pass filter 14 in front of the camera to block excitation light. The system may also include one or more filters 15 to select the wavelength range of the photo-excitation. With relatively thin samples it is also possible to have the excitation source 6 and camera 10 on opposite sides of the sample 4 as shown in FIG. 2, in which case the sample itself can serve as a long-pass filter. However a long-pass filter 14 may still be required if a significant amount of stray excitation light, reflected for example off other components, is reaching the camera. Either way, the acquired PL image can be analysed with a computer 16, using techniques disclosed for example in published PCT patent application Nos WO 2008/014537 A1, WO 2009/026661 A1 and WO 2009/121133 A1, to obtain information on average or spatially resolved values of a number of sample properties including minority carrier diffusion length, minority carrier lifetime, dislocation defects, impurities and shunts, amongst others, or on the incidence or growth of cracks. In principle the entire process can be performed in a matter of seconds or fractions of a second depending on factors such as the quality of silicon material and the readout speed of the camera, which is a timescale generally compatible with current solar block, cell and wafer production lines where, for example, the throughput for wafer and cell lines is of order one cell per second or two, and for block production where 30 seconds is typically available for the measurement of a full block face.
However the current PL imaging system as described above suffers from a number of disadvantages.
One disadvantage is that currently available PL imaging systems require samples to be removed from production lines and taken to the PL imaging tool, for example using robotic or manual pick and place handling. Manual pick and place handling is a labour intensive and slow process that often adds cost as well as being slow, and although robotic pick and place handling systems using platens or suction cups or similar are somewhat faster, they also add cost. Either way, the limited speed means only a small sample of the product in process can be tested. It would be beneficial to be able to measure all or most of the work product.
A further disadvantage is that in current PL imaging systems the sample has to be stationary during the measurement to prevent blurring of the image. A blurred image can prevent or compromise the capture of spatially resolved characterisation data, complicating the design and/or incorporation of a PL imaging system into production lines that are increasingly operating in continuous mode without stopping. To explain, with broad area 1 Sun excitation the photoluminescence emitted from many silicon samples, and raw or unpassivated silicon samples in particular, can be of such low intensity that even the most sensitive commercially available silicon-based CCD cameras require an exposure time of order at least 0.5 second to acquire a sufficient PL signal.
Yet another disadvantage with current PL imaging systems is the common reliance on laser excitation sources, typically in the near IR region of the spectrum. To explain, obtaining a measurable PL signal from low photoluminescence quantum efficiency samples such as raw or unpassivated silicon wafers and blocks (with quantum efficiency of order 10−6) often requires illumination intensities of 0.1 Watts/cm2 (˜1 Sun) or greater. A total optical power of tens of Watts is therefore required to illuminate silicon solar cell wafers that may typically be 15.6×15.6 cm2 in area, and laser excitation sources are usually considered to be essential to provide the required spectral purity and beam shaping. Furthermore for silicon samples the excitation light is typically in the near IR region (750 to 1000 nm), which is potentially very harmful because the eye focuses near-infrared light onto the retina but its protective ‘blink reflex’ response is triggered only by visible light. The potential hazard of laser light sources arises from the fact that they may be much brighter than other light sources, where the brightness (in units of power per unit area per unit solid angle) may be defined for example as the optical power passing through an aperture (e.g. a laser output aperture) divided by the aperture area divided by the solid angle subtended by the optical beam in the far field. When an extremely bright light source is viewed with the eye, either directly or via intermediate optics such as a collimating lens, the image formed on the retina can be extremely intense, resulting in virtually instantaneous and permanent damage. However although there is less likelihood of this occurring with incoherent near IR light, e.g. from high power LEDs, it needs to be understood that because brightness is a key parameter, light safety issues cannot simply be ignored just because a system uses non-laser (incoherent) light sources.
Current PL imaging systems are therefore further complicated by light safety issues, since the PL measurement chamber generally must be optically isolated to avoid the risk of operators being exposed to high brightness IR light that could cause eye damage. This usually requires shutters, doors or equivalent mechanisms, adding complexity and cost to the sample transfer mechanisms into and out of the PL measurement chamber. Because of these complications, the basic PL imaging apparatus shown in FIG. 1 or 2 requires several modifications if it is to be used safely and cost effectively to characterise silicon solar cells on a production line.